The oxidation of hydrocarbons is an important reaction in industrial organic chemistry. Thus, for example, the oxidation of cyclohexane is used commercially to produce cyclohexanol and cyclohexanone, which are important precursors in the production of nylon, whereas the oxidation of alkylaromatic hydrocarbons is used to produce phenol, a precursor in the production of polycarbonates and epoxy resins.
Oxidation of hydrocarbons can be conducted using well-known oxidizing agents, such as KMnO4, CrO3 and HNO3. However, these oxidizing agents have the disadvantage of being relatively expensive, and moreover their use is accompanied by the production of unwanted coupling products which can represent disposal problems.
Preferably, therefore, oxidizing agents based on peroxides or N2O are used. The cheapest oxidizing agent, however, is molecular oxygen, either in pure form or as atmospheric oxygen. However, oxygen itself is usually unsuitable for oxidizing hydrocarbons, since the reactivity of the O2 molecule, which occurs in the energetically favorable triplet form, is not sufficient.
By using redox metal catalysts it is possible to utilize molecular oxygen for oxidizing organic compounds and hence a great number of industrial processes are based on the metal-catalyzed autooxidation of hydrocarbons. Thus, for example, the oxidation of cyclohexane with O2 to cyclohexanol and/or cyclohexanone proceeds with the use of cobalt salts. These industrial processes are based on a free-radical chain mechanism, in which the bi-radical oxygen reacts with a hydrocarbon free radical, with formation of a peroxy radical and subsequent chain propagation by abstraction of an H atom from a further hydrocarbon. In addition to metal salts, however, organic molecules can also act as free-radical initiators.
However, it is a disadvantage of these processes that the selectivity decreases greatly with increasing conversion and therefore the processes must be operated at a very low level of conversion. Thus, for example, the oxidation of cyclohexane to cyclohexanol/cyclohexanone is carried out at a conversion of 10 to 12% so that the selectivity is 80 to 85% (“Industrielle Organische Chemie” [Industrial Organic Chemistry] 1994, 261, VCH-Verlag, D-69451 Weinheim).
An alternative to metal salt catalysts are organic mediators, for example N-hydroxyphthalimide (NHPI). Thus, U.S. Pat. Nos. 6,852,893 and 6,720,462 describe methods for oxidizing hydrocarbon substrates by contacting the substrate with an oxygen-containing gas, in which the oxygen content is from 5 to 100% by volume, in the presence of a free radical initiator and a catalyst, typically an N-hydroxycarbodiimide catalyst, such as N-hydroxyphthalimide (NHPI). The process is conducted at a temperature between 0° C. and 500° C. and a pressure between atmospheric and 100 bar (100 and 10,000 kPa). The molar ratio of the catalyst to the hydrocarbon substrate can range from 10−6 mol % to 1 mol %, whereas the molar ratio of free-radical initiator to the catalyst can be 4:1 or less, such as 1:1 to 0.5:1. Suitable substrates that may be oxidized by this process include cumene, cyclohexylbenzene, cyclododecylbenzene and sec-butylbenzene.
U.S. Pat. No. 7,038,089 discloses a process for preparing a hydroperoxide from a hydrocarbon selected from the group consisting of primary hydrocarbons, secondary hydrocarbons and mixtures thereof, which process comprises oxidizing the hydrocarbon at a temperature in the range between 130 and 160° C. with an oxygen-containing gas in a reaction mixture containing the hydrocarbon and a catalyst comprising a cyclic imide compound and an alkali metal compound. Suitable hydrocarbons are said to include C4 to C20 tertiary alkanes (e.g., iso-butane, iso-pentane, iso-hexane, and the like), C7 to C20 (alkyl) aromatic hydrocarbons with 1 to 6 aromatic rings or C9 to C20 (cycloalkyl) aromatic hydrocarbons with 1 to 6 aromatic rings (e.g., xylene, cumene, cymene, ethylbenzene, diisopropylbenzene, cyclohexylbenzene, tetrahydronaphthalene (tetraline), indan, etc.). The amount of the cyclic imide compound used may be adduct or salt from 0.0001 to 1%, preferably from 0.0005 to 0.5%, by weight based on the reaction mixture, whereas the amount of the alkali metal compound may be from 0.000005 to 0.01%, preferably from 0.00001 to 0.005%, by weight based on the reaction mixture.
However, although current work has continued to demonstrate the utility of cyclic imides as hydrocarbon oxidation catalysts, it has also shown that their application in a commercial process requires further development. In particular, cyclic imides, such as N-hydroxyphthalimide, are expensive and are readily hydrolyzed under the conditions of the oxidation reaction. Moreover, unreacted imide catalysts and their decomposition products (acids and ethers) can pose significant problems to downstream reactions, such as hydroperoxide cleavage. Thus the successful application of cyclic imides to the oxidation of hydrocarbons will require treatment of the oxidation effluent to remove unreacted imides and their decomposition products and, if possible, recovery of the valuable unreacted imides.
International Application No. WO 2009/025939 discloses a process for oxidizing a hydrocarbon to the corresponding hydroperoxide, alcohol, ketone, carboxylic acid or dicarboxylic acid, in which the hydrocarbon is contacted with an oxygen-containing gas in the presence of a catalyst comprising a cyclic imide to produce an effluent comprising an oxidized hydrocarbon product and unreacted imide catalyst. The effluent is then treated with an aqueous solution of a base, particularly an alkali metal carbonate and/or hydrogen carbonate, to produce an aqueous phase comprising a salt of the unreacted imide and an organic phase comprising the oxidized hydrocarbon product. The aqueous phase is then acidified to precipitate the unreacted imide catalyst, which is then recovered and recycled to the oxidation step.
According to the present invention, it has now been found that as an alternative, or in addition, to capturing the unreacted imide with a base and recycling the imide to the hydrocarbon oxidation step, it is possible to feed the captured imide to an imide manufacturing process and thereby enhance the imide yield of the process.